The present invention relates to covered rolls used in papermaking operations and methods of producing the same. More specifically, the invention relates to covered rolls used in papermaking operations which contain a polymeric coating material which is more easily removed from the roll upon which it is processed.
Cylindrical rolls are utilized in a number of industrial applications, especially those relating to papermaking. Such rolls are typically employed in demanding environments in which they can be exposed to high dynamic loads and temperatures. As an example, in a typical paper mill, rolls are used not only for transporting a fibrous web sheet between processing stations, but also, in the case of pressure and calendar rolls, for processing the web sheet itself into paper.
Pressure rolls are included, inter alia, in the press section of a papermaking machine in which press felts apply pressure to a newly formed web and, through heat and pressure, remove moisture from the web. Typically, the web is conveyed on the press felt through a nip between two pressure rolls, which tends to squeeze moisture from the web. In this environment, such pressure rolls are subjected to high dynamic loads due to the extreme pressures and high paper speeds, and also to moisture. Accordingly, the rolls should be strong, tough, wear-resistant, and moisture-resistant. Also, often the rolls are xe2x80x9ccrownedxe2x80x9d along the longitudinal axis for to reduce wrinkling of the web.
Calender rolls are often employed in the calendering section of a papermaking machine. Typically, a series of calender rolls are positioned to define a somewhat convoluted path for the paper web, with some of the rolls located closely enough to one another that they apply pressure to the paper web. Calendering is performed primarily to improve the smoothness and gloss of the paper, each of which are affected by the number and surface hardness of the calender rolls and pressure applied thereby.
Because papermaking rolls can have many different performance demands, and because replacing an entire metallic roll can be quite expensive, many papermaking rolls include a polymeric cover that surrounds the circumferential surface of a metallic core. By varying the polymer employed as a cover, the designer can provide the roll with different performance characteristics as the papermaking application demands. Also, replacement of a cover over a metallic roll can be less expensive than the replacement of an entire metallic roll. The polymers used in the cover are most often thermosets or thermoplastics.
In addition, some rolls comprise a polymeric shell core. In these rolls, the shell is typically supported by an internal frame which includes hydraulic or pneumatic cylinders that press on the inner surface of the shell. In manufacturing the cover shell for a roll, generally a polymer is applied to a core formed of metal or some other rigid material, cast or molded into the desired shape, and cured. The cover or shell is then removed from the mold core and bonded to a core or internal frame.
Problems arise in the creation of polymer covers or shells due to the marked difference in thermal expansion between the polymeric material and the material of the core. More specifically, the polymeric material typically has a coefficient of thermal expansion which is an order of magnitude greater than that of the metal. As a result, the sleeve formed from the polymeric material tends to shrink, and thus closely clings or sticks to the mold core. Accordingly, it is often very difficult to remove the polymeric sleeve from the mold core. Additionally, due to the shrinkage of the polymeric material, undesirable residual stresses may form therein.
There remains a need in the art to provide polymeric covers and shells which are more easily removable from the core molds upon which they are made.
In view of the foregoing, it is an object of the present invention to provide a covered roll structure for use in papermaking which allows a sleeve of polymeric material formed thereon to be more easily removed from the roll structure.
It is another object of the present invention to provide a method of producing such a covered roll structure.
To this end and others, the present invention provides covered roll structures which allow the sleeve of polymeric material formed thereon to be more easily removed from roll structures than conventional polymeric sleeves. Once removed, the sleeve of polymeric material can be used to cover another core to form a roller or used as a shell by itself or with an internal frame.
In one embodiment, a roll structure is formed by first applying a removable material, preferably formed from an inorganic pre-impregnated powder, to a mold core having a cylindrical outer surface to form a sleeve of removable material. The removable material preferably has a melting point of at least about 100xc2x0 F. Subsequently, polymeric material is applied over the sleeve of removable material to form a sleeve of polymeric material. The sleeve of polymeric material is subsequently cured, and the removable material is dissolved or otherwise removed from the roll structure. As a result, the sleeve of polymeric material, which can serve as either a cover or a shell, can be readily removed from the roll structure.
In another embodiment, a sleeve of compressible material may be formed subsequent to the formation of the sleeve of removable material and prior to the formation of the sleeve of polymeric material. Advantageously, the compressible material is able to contract during and after the curing process such that the polymeric material may be able to shrink to a greater extent than if removable material were present without the compressible material. This structure can reduce the amount of stress present in the cover or shell. If desired, the sleeve of polymeric material may be readily removed from the roll structure and attached to a roll core or frame. Alternatively, the sleeve can be bonded to the mold core, which also serves as the core for the operational roll itself. The compressible material can also serve as a conduit for solvent if the removable material is to be removed by dissolution with that solvent.